Not personally, but on a (delayed) MAD-LHR last year I was in the front cabin of the 757 (but behind the curtain!). There was a loud thud from behind the galley and then the crew rushing around with oxygen, Deli Boxes, etc. for a few minutes. I was close so could overhear once the chap came around and he said he'd not eaten all day and thought it was just low blood sugar. No major drama (thankfully!).
Phil